Chemistry 100 Bettelheim, Brown, Campbell & Farrell Ninth Edition Introduction to General, Organic and Biochemistry Chapter 2 Atoms Classifications of Matter: Elements An Element is a substance (for example, carbon, hydrogen, and iron) that consists of atoms all of which have the same Atomic Number, i.e. number of protons. There are 114 known elements. 88 of these elements occur in nature; the others have been made by chemists and physicists. Their symbols consist of one or two letters. Their names (& symbols) are derived from a variety of sources: the English or other language name of the element, people important in atomic science, geographic locations, planets, mythology, etc. Classifications of Matter: Compounds A Compound is a pure substance made up of two or more elements in a fixed ratio by mass. The Formula of a compound tells us the ratios of its constituent elements and identifies each element by its atomic symbol. Sodium Chloride has the formula NaCl. This means the ratio of sodium atoms to chlorine atoms in sodium chloride is 1:1 Water has the formula H 2 O. This means the ratio of hydrogen atoms to oxygen atoms in water is 2:1. 1
Classifications of Matter: Mixtures A Mixture is a combination of two or more pure substances. A mixture may be homogeneous or heterogeneous. The substances in a mixture may be present in any mass ratio. Each substance in a mixture has a different set of physical properties. If the physical properties of the individual components of the mixture are known, we can use appropriate physical means to separate the mixture into its component parts. Dalton s Atomic Theory All matter is composed of very tiny particles, which Dalton called atoms. All atoms of the same element have the same chemical properties. A molecule is a tightly bound combination of two or more of the same or different kinds of atoms that act as a unit. Compounds are formed by the chemical combination of two or more different kinds of atoms. Evidence for Daltons Atomic Theory Monatomic elements -These elements exist as single atoms. Examples are: helium (He) and neon (Ne). Diatomic elements - There are seven elements that occur as diatomic molecules: H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 Polyatomic elements - Some elements have three or more atoms per molecule. O 3, P 4, S 8 Many elements, like diamond, have millions of atoms bonded together to form a gigantic cluster or network. 2
The Structure of Atoms Once atoms were thought to be undividable, this has since been shown to be not true. Atoms are composed of Sub-Atomic Particles: Subatomic particle Proton Electron Neutron Charge +1-1 0 Mass (g) Mass (amu) 1.6726 x 10-24 1.0073 1 9.1094 x 10-28 5.4858 x 10-4 0.0005 1.6749 x 10-24 1.0087 1 Mass (amu); to one significant Location in figure an atom In the nucleus Outside the ucleus In the nucleus The unit of mass, the atomic mass unit (amu) is defined as 1/12 the mass of an atom of carbon with 6 protons and 6 neutrons in its nucleus. 1 amu = 1.6605 x 10-24 g Atomic Number & Mass Number The Atomic Number is the number of protons in the nucleus of an atom. This number defines the element. The Mass number is the sum of the number of protons plus neutrons in the nucleus of an atom. Many elements have atoms with different masses! These are called Isotopes. The mass of the electrons in an atom is so small compared to that of its protons and neutrons that electrons are not counted in determining mass number. 12 #p + #n = mass number carbon-12: 6C #p = atomic number Isotopes Isotopes are atoms of an element, i.e. have the same number of protons, but which have a different number of neutrons in their nucleus: carbon-12 has 6 protons and 6 neutrons; carbon-13 has 6 protons and 7 neutrons; carbon-14 has 6 protons and 8 neutrons; Most elements found on Earth are mixtures of isotopes: chlorine is 75.77% chlorine-35 and 24.23% chlorine-37 Only a few elements are mono-isotopic: Fluorine-19 3
The Size & Mass of Atoms Consider an atom of lead-208 It has 82 protons, 82 electrons, and 126 neutrons It has a mass of 3.5 x 10-22 g It requires 1.3 x 10 24 atoms to make 1 lb of lead-208 The diameter of the nucleus is 1.6 x 10-14 m The diameter of the atom is 3.5 x 10-10 m The density of the atom is 11.3 g/cm 3 The density of the nucleus is 1.8 x 10 14 g/cm 3 The Periodic Chart of the Elements Dmitri Mendeleev (1834-1907) He arranged the known elements in order of increasing atomic weight beginning with hydrogen. He observed that when elements are arranged in this manner, certain sets of properties recur periodically. He then grouped those elements with recurring sets of properties in the same columns. Li, Na, and K were arranged in the same column, a group or family, on the left most part. Each starts a new period, a horizontal row. Classification of the Elements Most elements are Metals they are solids (except for mercury, Hg, which is a liquid), shiny, conductors of electricity, ductile, and malleable. Most metals dissolve in each other to form alloys; brass is an alloy of copper and zinc. They tend to give up electrons in their chemical reactions. Right-most elements are Nonmetals, only hydrogen (H), lies on the left side of the Periodic Table. Except for graphite, nonmetals do not conduct electricity. Nonmetals tend to accept electrons in their chemical reactions. 4
Classification of the Elements In between the metals and nonmetals are the Metalloids which have intermediate properties to those found in the other classes. These six elements are: boron, silicon, germanium, arsenic, antimony, and tellurium. They have some of the properties of metals and some of nonmetals; for example, they are shiny like metals but do not conduct electricity. One of the metalloids, silicon, is a semiconductor; it does not conduct electricity under certain applied voltages, but becomes a conductor at higher applied voltages. Impurities also have an enormous effect. Why Elements have Periodic Properties Electron Configurations Electron configuration: the arrangement of electrons in an atom in the space outside the nucleus. The energy of electrons in an atom is quantized, which means that an electron in an atom can have only certain allowed energies. This is Quantum Mechanics! The Ground State of an atom is the electron configuration of lowest energy. A similar ground state re-occurs periodically in quantum mechanics. Electron Configurations Shell Orbitals contained in each shell 4 one 4s, three 4p, five 4d, and seven 4f orbitals 3 one 3s, three 3p, and five 3d orbitals 2 one 2s and three 2p orbitals 1 one 1s orbital Maximum number of electrons shell can hold 2 + 6 + 10 + 14 = 32 2 + 6 + 10 = 18 2 + 6 = 8 2 As atoms grow larger, they have more shells, each shell adds a sub-shell to a copy of what the lower shell had. It is this recurrence of sub-shells that causes the periodicity of properties Mendeleev observed. These recurring outer electron configurations at higher energies gives us similar but unique properties of elements. 5
Electron Configurations Electron configurations are governed by three rules: Rule 1: Orbitals fill in the order of increasing energy from lowest to highest. The Aufbau principle. Rule 2: Each orbital can hold up to two electrons with their spins paired. The Pauli Exclusion principle. Rule 3: When there is a set of orbitals of equal energy, each orbital becomes half filled before any of them becomes completely filled. This called Hund s rule. Energy Increasing 7p x 7p y 7p z 7s Electron Configurations [?] [Rn] 6d 5f 6s 6p x 6p y 6p z [Xe] 5d 4f 5s 4s 5p x 5p y 5p z [Kr] 4p x 4p y 4p z [Ar] 4d 3d What is the electron configuration of this element? 3p x 3p y 3p z 3s [Ne] 2p x 2p y 2p z 2s [He] What is the Noble Gas notation for the electron configuration? 1s Ionization Energy Ionization energy is the energy required to remove the most loosely held electron from an atom in the gaseous state. Example: When lithium loses one electron, it becomes a lithium ion; it still has three protons in its nucleus, but now only two electrons outside the nucleus. It therefore acquires a positive charge and becomes a lithium ion. I.E. Li Li A metallic element can lose electrons up to three commonly, occasionally four, or until it reaches the noble gas electron configuration. Each additional electron takes more energy to remove than the last. 6